This invention relates to a method of making a silver halide photographic material containing at least one silver halide emulsion that has enhanced light absorption. The invention is directed in particular to a method of making an emulsion with high sensitivity, reduced fog and granularity.
J-aggregating cyanine dyes are used in many photographic systems. It is believed that these dyes adsorb to a silver halide emulsion and pack together on their xe2x80x9cedgexe2x80x9d which allows the maximum number of dye molecules to be placed on the surface. However, a monolayer of dye, even one with as high an extinction coefficient as a J-aggregated cyanine dye, absorbs only a small fraction of the light impinging on it per unit area. The advent of tabular emulsions allowed more dye to be put on the grains due to the increased surface area per mole of silver. However, in most photographic systems, it is still the case that not all of the available light is being collected.
The need is especially great in the blue spectral region where a combination of low source intensity and relatively low dye extinction results in a deficient photoresponse. The need for increased light absorption is also great in the green sensitization of the magenta record of multilayer color film photographic elements. The eye is most sensitive to the magenta image dye and this layer has the largest impact on color reproduction. Higher speed in this layer can be used to obtain improved color and image quality characteristics. The cyan layer could also benefit from increased red-light absorption that could allow the use of smaller emulsions with less radiation sensitivity and improved color and image quality characteristics. For certain applications, it may be useful to enhance infrared light absorption in infrared sensitized photographic elements to achieve greater sensitivity and image quality characteristics.
One way to achieve greater light absorption is to increase the amount of spectral sensitizing dye associated with the individual grains beyond monolayer coverage of dye (some proposed approaches are described in the literature, G. R. Bird, Photogr. Sci. Eng., 18, 562 (1974)). One method is to synthesize molecules in which two dye chromophores are covalently connected by a linking group (see U.S. Pat. Nos. 2,518,731, 3,976,493, 3,976,640, 3,622,316, Kokai Sho 64(1989)91134, and EP 565,074). This approach suffers from the fact that when the two dyes are connected they can interfere with each other""s performance, e.g., not aggregating on or adsorbing to the silver halide grain properly.
In a similar approach, several dye polymers were synthesized in which cyanine dyes were tethered to poly-L-lysine (U.S. Pat. No. 4,950,587). These polymers could be combined with a silver halide emulsion, however, they tended to sensitize poorly and dye stain (an unwanted increase in D-min due to retained sensitizing dye after processing) was severe in this system and unacceptable.
A different strategy involves the use of two dyes that are not covalently linked to one another. In this approach the dyes can be added sequentially and are less likely to interfere with each other. Miyasaka et al. in EP 270 079 and EP 270 082 describe silver halide photographic material having an emulsion spectrally sensitized with an adsorbable sensitizing dye used in combination with a non-adsorbable luminescent dye that is located in the gelatin phase of the element. Steiger et al. in U.S. Pat. Nos. 4,040,825 and 4,138,551 describe a silver halide photographic material having an emulsion spectrally sensitized with an adsorbable sensitizing dye used in combination with a second dye that is bonded to gelatin. The problem with these approaches is that unless the dye that is not adsorbed to the grain is in close proximity to the dye adsorbed on the grain (less than 50 angstroms separation) efficient energy transfer will not occur (see T. Fxc3x6rster, Disc. Faraday Soc., 27, 7 (1959)). Most dye off-the-grain in these systems will not be close enough to the silver halide grain for energy transfer, but will instead absorb light and act as a filter dye leading to a speed loss. A good analysis of the problem with this approach is given by Steiger et al. (Photogr. Sci. Eng., 27, 59 (1983)).
A more useful method is to have two or more dyes form layers on the silver halide grain. Penner and Gilman described the occurrence of greater than monolayer levels of cyanine dye on emulsion grains, Photogr. Sci. Eng., 20, 97 (1976); see also Penner, Photogr. Sci. Eng., 21, 32 (1977). In these cases, the outer dye layer absorbed light at a longer wavelength than the inner dye layer (the layer adsorbed to the silver halide grain). Bird et al. in U.S. Pat. No. 3,622,316 describe a similar system. A requirement was that the outer dye layer absorb light at a shorter wavelength than the inner layer. A problem with previous dye layering approaches was that the dye layers described produced a very broad sensitization envelope. This may be desirable for some black and white photographic applications, but in a multilayer color film element this would lead to poor color reproduction since, for example, the silver halide grains in the same color record would be sensitive to both green and red light.
Yamashita et al. (EP 838 719 A2, U.S. Pat. No. 6,117,629) describes the use of two or more cyanine dyes to form more than one dye layer on silver halide emulsions. The dyes are required to have at least one aromatic or heteroaromatic substituent attached to the chromophore via the nitrogen atoms of the dye. Yamashita et al. teaches that dye layering will not occur if this requirement is not met. This is undesirable because such substitutents can lead to large amounts of retained dye after processing (dye stain) that affords increased D-min. Similar results are described in U.S. Pat. No. 6,048,681 and EP 1,061,431A1. EP 1,061,411 A1 describes forming dye layers by using dyes with additional polycyclic rings. The dyes have at least one heterocyclic ring that has two or more additional rings attached to it. This may promote dye-dye interactions by increasing van der Waals forces, however, adding hydrophobic, aromatic rings to the dye molecules is undesirable in that the dyes are more likely to be retained after processing and give higher dye stain. Yamashita and Kobayashi (JP 10/171058) describe silver halide photographic emulsions that contain an anionic dye and a cationic dye, where the charge of either the anionic dye or the cationic dye is 2 or greater. Tadashi and Takashi describe (JP2001013614A) combinations of cyanine dyes wherein the logP for the dye combination is in a certain preferred range.
Further improvements in dye layering have been described in U.S. Pat. Nos. 6,143,486, 6,165,703, 6,329,133, 6,331,385, and 6,361,932. Useful antenna dyes (dyes in the outer layer of the multilayer) for dye layering that have less dye stain after processing were described in U.S. Pat. No. 6,312,883.
It also known in the art to add a scavenger for oxidized developer to a photographic element in order to prevent oxidized developing agent from reacting within the element at an undesired location or at an undesired point in time. In particular, it is undesirable for oxidized developer to diffuse away from the imaging layer in which it formed and into other color records where it can form dye in the wrong layer. Thus, scavengers for oxidized developer are typically located in non-image forming interlayers between two imaging layers. However, in some situations early formation of dye can have an undesirable impact on tone scale and fog formation. Thus, it is also known to add scavengers for oxidized developers directly to imaging layers in order to modulate Dox levels.
Typically, scavengers reduce or eliminate oxidized developers without forming any permanent dyes. They also do not cause stains nor release fragments that have photographic activity. They are also typically rendered substantially immobile in the element by incorporation of an anti-diffusion group (a ballast) or by attachment to a polymer backbone.
Known scavengers for oxidized developers include ballasted para hydroquinone (1,4-dihydroxybenzene) compounds such as described in U.S. Pat. Nos. 3,700,453, 4,732,845, 5,561,036, 6,045,988 and 5,585,230; ballasted gallic acid (1,2,3-trihydroxybenzene) compounds as described in U.S. Pat. Nos. 4,474,874 and 4,476,219; ballasted resorcinol (1,3-dihydroxybenzene) compounds as described in U.S. Pat. Nos. 3,770,431, 5,856,072 and 3,772,014; ballasted hydrazides such as described in U.S. Pat. Nos. 4,923,787, 4,971,890, 5,147,764, 5,164,288, 5,230,992, 5,629140 and 5,543,277; ballasted pyrocatechol (1,2-dihydroxybenzene) compounds as described in U.S. Pat. Nos. 4,175,968, 5,561,036, 4,252,893, 5,561,035 and DE766,135; couplers which do not form permanent dyes such as those described in U.S. Pat. Nos. 5,932,407, 5,629,140, EP 0284099 and U.S. Pat. No. 6,013,428; and disulfonamidophenyl scavengers as described in U.S. Pat. Nos. 4,447,523, 4,205,987, 4,717,651, 5,478,712 and 6,255,045.
It is known that water-solubilizing groups may be used to increase the reactivity towards Dox in many of these classes of scavengers. Addition of water-solubilizing groups to ballasted compounds tend to impart surfactant-like properties to the material. However, for the types of emulsions and formats used in the above references, the additional surfactant-like properties of ballasted scavengers with water solubilizing groups do not confer any additional advantages or utility.
It is also know in the art to utilize various surfactants in photographic elements for many different reasons including, for example, surface-tension control to prevent stacked liquid layers from mixing during multiplayer coating processes. The art in this area is voluminous, but is generally discussed in Research Disclosure, September 1996, Item 38957.
Dye-layered silver halide emulsions using cationic antenna sensitizing dyes provide enhanced light absorption and photographic sensitivity (speed) in photographic elements. However, as currently practiced, these materials often produce concomitant unacceptable increases in silver fog and associated granularity which may limit their practical utility. This problem is often exacerbated with the use of smaller emulsions and especially so with emulsion sizes of 1 micron (equivalent circular diameter), or less. Silver halide antifoggants such as N-(3-(2,5-dihydro-5-thioxo-1H-tetrazole-1-yl)phenyl)-Acetamide (APMT) and 5-methyl-(1,2,4)Triazolo(1,5-a)pyrimidin-7-ol, sodium salt (TAI) have been disclosed for use with dye-layered emulsions, but are insufficient to completely reduce the D-min. Increasing amounts of APMT reduce the fog, but cause unacceptable loss of emulsion sensitivity. Other common antifoggants and stabilizers were found to be ineffective for minimizing silver fog and its associated granularity signal without large speed loss. It remains a problem to achieve both high sensitivity and low fog in a dye-layered emulsion.
In one embodiment this invention provides a method of spectrally sensitizing a silver halide emulsion comprising the following steps in the following order
a) providing a silver halide emulsion comprising tabular silver halide grains having an inner dye layer adjacent to the silver halide grain, said dye layer comprising at least one dye (Dye 1) that is capable of spectrally sensitizing silver halide,
b) adding to the emulsion at least one dye (Dye 2) capable of providing a second dye layer adjacent to the inner dye layer, and
c) adding to the emulsion a non-cationic surfactant,
to form a silver halide emulsion comprising silver halide grains having associated therewith two dye layers, wherein the dye layers are held together by non-covalent forces or by in situ bond formation; the outer dye layer adsorbs light at equal or higher energy than the inner dye layer; and the energy emission wavelength of the outer dye layer overlaps with the energy absorption wavelength of the inner dye layer.
In another embodiment this invention provides a method of spectrally sensitizing a silver halide emulsion comprising the following steps in the following order
a) providing a silver halide emulsion comprising tabular silver halide grains having an inner dye layer adjacent to the silver halide grain, said dye layer comprising at least one dye (Dye 1) that is capable of spectrally sensitizing silver halide,
b) adding to the emulsion at least one dye (Dye 2) capable of providing a second dye layer adjacent to the inner dye layer, and
c) adding to the emulsion a scavenger for oxidized developer,
to form a silver halide emulsion comprising silver halide grains having associated therewith two dye layers, wherein the dye layers are held together by non-covalent forces or by in situ bond formation; the outer dye layer adsorbs light at equal or higher energy than the inner dye layer; and the energy emission wavelength of the outer dye layer overlaps with the energy absorption wavelength of the inner dye layer.
In a preferred embodiment both a scavenger for oxidized developer and a surfactant are added during step c). Silver halide photographic elements containing emulsions made as described herein exhibit both high sensitivity and low fog. Such elements also exhibit reduced granularity.